More than a century ago, Golgi suggested that glial cells provide nutrients to neurons. This idea gained credibility when astrocytes, but not neurons, were discovered to contain glycogen, the main energy reserve in the brain. Recent observations on the retinas of the honeybee and rabbit, have provided modem experimental evidence that glial cells can provide fuel to neurons. The role of glycogen, however, remains a mystery. The experiments proposed in this application will critically address the role of glycogen in glial- neuronal interactions during brain energy metabolism. These experiments will be carried out using an advantageous preparation of central nervous system (CNS) white matter, the isolated rat optic nerve. The long term goals of this research are: 1) to learn more about the physiology and function of glial cells, and 2) to understand the mechanisms of CNS white matter injury as occurs with stroke, hypoglycemia, anoxia or trauma, and to devise better ways of minimizing this injury. Broadly stated, the specific aims of this proposal are to learn how glial cells and axons in the CNS interact when deprived of glucose and to understand how glucose deprivation injures central axons. Two hypotheses will be tested: 1) During hypoglycemia in the CNS, astrocytes supply energy substrate to axons in the form of lactate derived from glycogen. Axon function and survival depend on glycogen in the absence of glucose. 2) Axon injury caused by glucose deprivation is Ca2+- dependent and is due to Ca2+ entry mediated by reverse Na+/Ca2+ exchange and Ca channels. The role of astrocytes in supplying energy substrate to axons will be studied using the in vitro rat optic nerve preparation, quantitative electrophysiological techniques to monitor optic nerve function, chemical measurement of glycogen content, and pH, glucose-sensitive, and lactate sensitive microelectrodes. The mechanisms of hypoglycemia-induced axon injury will be studied using quantitative measures of optic nerve function in conjunction with measurement of [Ca2+ ]o using Ca2+-sensitive microelectrodes. These studies will provide useful new information about the manner in which glial cells and neurons interact in the context of brain energy metabolism. They will also assess the importance of astrocytic glycogen for neural function during and for recovery after periods of hypoglycemia, and may suggest novel strategies to stave off neural injury under these conditions.